Various proposals have been made in regard to the technology of front and rear wheel steering devices which improve the handling of the vehicle by steering the rear wheels in a certain relationship to the front wheel steering angle (Japanese patent laid open publication No. 9-58515). Such a front and rear wheel steering device is typically incorporated with a fail-safe device for automatically restoring the steering angle to the neutral position (straight ahead position) in case of a failure of the steering sensor (or upon stopping the engine).
According to a known fail-safe device, a return spring is used for forcing the steering rod back to the neutral position when a normal steering angle control is not possible (see Japanese patent laid open publication No. 8-301131, for instance).
However, according to this conventional device, because the steering rod is constantly subjected to the spring force of a return spring which is powerful enough to restore the steering to the neutral position and allow the vehicle to move straight ahead even when the steering sensor has failed and it is no longer possible to determine the neutral position, the motor for producing the steering angle must be powerful enough to oppose the spring force of the return spring in addition to overcoming the resistance from the road surface. This factor has prevented a compact design of the electric motor and the drive circuit for the motor.
Japanese patent laid open publication No. 5-69834 discloses an arrangement which comprises a primary and secondary drive units, and clutches for selectively coupling the drive units to the steering device so that the rear wheel steering angle may be restored to the neutral position with the secondary steering device in case of a failure of the primary steering device. However, the steering angle sensor must be able to determine the neutral position at all times, and the need for the secondary drive unit which is needed only at the time of a failure undesirably adds to the size of the overall system.